1. Field of the Invention
The invention relates to an injection molding machine that achieves molding by injecting a material into a mold. In particular, the invention relates to a device for adjusting the position of a screw in the axial direction, which plasticizes or mixes the material.
2. Description of Related Art
Patent Literature 1 discloses an injection molding machine, which separately includes a screw for plasticizing resin and an injection shaft for injecting molten resin into a mold. In this injection molding machine, thermoplastic resin is plasticized by rotation of the screw in the plasticizing cylinder. By advancing a plunger that serves as the injection shaft in the injection cylinder, the molten resin is injected into the mold. A driving device of the screw includes a rotating device composed of an electric or hydraulic motor for rotating the screw. The driving device further includes a hydraulic actuator for reciprocating the screw in the axial direction thereof. The plasticizing cylinder has a plasticization chamber and the injection cylinder has an injection chamber. The plasticization chamber communicates with the injection chamber through a communicating passage. In the process of plasticizing the resin, the molten resin is fed into the injection cylinder through the communicating passage. Prior to the process of injecting the molten resin, the communicating passage is closed by the conical front face of the screw. Thus, the screw is able to reciprocate in the axial direction.
Patent Literature 2 discloses a liquid resin injection molding machine that separately includes a screw for mixing two types of materials and an injection shaft for injecting the mixed material into the mold. In the liquid resin injection molding machine, two types of liquid resins in a mixing cylinder are mixed by rotation of the screw. By advancing a plunger that serves as the injection shaft in the injection cylinder, the mixed liquid is injected into the mold. The mixing cylinder has a mixing chamber and the injection cylinder has an injection chamber. The mixing chamber communicates with the injection chamber through a communicating passage. To prevent the mixed liquid from flowing back to the mixing cylinder from the injection cylinder, the screw can be advanced in the axial direction to close the communicating passage with its conical front face. Hereinafter, the plasticizing cylinder and the mixing cylinder are collectively referred to as a material supply cylinder.
According to Patent Literatures 1 and 2, the screw that plasticizes or mixes the material reciprocates between a retracted position and an advanced position. The retracted position is a position where the screw is away from the communicating passage. The screw rotates in the retracted position to send the material into the injection cylinder. The advanced position is a position where the front face of the screw closes the communicating passage. When the screw advances to close the communicating passage, the molten resin before the screw flows into the injection cylinder. However, part of the molten resin before the screw may flow backward and does not enter the injection cylinder. The amount of the resin that flows back may vary depending on the viscosity of the molten resin. As a result, there is a measuring error between the amount of the material that is actually fed into the injection cylinder from the material supply cylinder and a set value. As the distance between the retracted position and the advanced position increases, the measuring error also increases. For this reason, it is preferable to reduce the distance between the retracted position and the advanced position.
Patent Literature 3 discloses a screw position adjusting device that includes a connecting shaft coupled to the rear end of the screw. The connecting shaft connects the screw with the screw rotating device. The screw is movable in the axial direction but non-rotatable with respect to the connecting shaft. The connecting shaft has a hole through which an adjustment rod penetrates the connecting shaft coaxially. A female thread portion is formed on the inner peripheral surface of the hole while a male thread portion is formed on the outer peripheral surface of the adjustment rod. The adjustment rod is screwed to the connecting shaft. A fastener for preventing loosening of the adjustment rod is provided at the rear end of the adjustment rod. The front end of the adjustment rod is in contact with the rear end of the screw. The retracted position of the screw is adjustable through rotation of the adjustment rod.
Patent Literature 4 discloses another screw position adjusting device. The rear end portion of the screw is spline-coupled to a piston member, and the piston member is fitted to a cylinder member. The cylinder member is fixed to the screw rotating device. A first flange is formed on the plasticizing cylinder, and a second flange is formed on the cylinder member. A ring member is screwed to the outer periphery of one of the first flange and the second flange. A gap between the first and second flanges is adjustable through rotation of the ring member. The gap between the first and second flanges determines the retracted position of the screw. The second flange is fastened to the first flange by a bolt.